Sodium ion conducting sodium aluminum phosphate glasses

ABSTRACT

Certain sodium aluminum phosphate glasses containing up to 11 mole percent of ZrO 2 , MgO or SiO 2  are good ionic conductors and are resistant to corrosion by molten sodium at about 300° C. Exemplary is a glass consisting of 50 mole percent Na 2  O, 38 mole percent P 2  O 5 , 12 mole percent Al 2  O 3  2 mole percent MgO and 4 mole percent SiO 2 .

DESCRIPTION

1. Technical Field

This invention relates to the discovery that certain glasses in the Na₂ O--Al₂ O₃ --P₂ O₅ system are good sodium ion conductors and are useful as solid electrolytes in electrochemical devices.

2. Background Art

Solid ionic conductors with Na³⁰ as the mobile species are desirable for use as solid electrolytes.

Sodium has long been regarded as a desirable component of galvanic cells. It is inexpensive and its high reduction potential and light weight have often suggested its use as the anodic component in high energy-density storage batteries. Na⁺ conductors such as β--Al₂ O₃ which are solid and are highly conductive at low temperatures are quite useful in such utilities; these conductors also serve to separate the anodic and cathodic components. (See, for example, J. T. Kummer, Prog. in Solid State Chem., 7, 141 (1972), J. T. Kummer and N. Weber, "A Sodium-Sulfur Secondary Battery" SAE paper No. 670179, Jan. 1967, and L. S. Marcous and E. T. Seo, "Sodium-Sulfur Batteries", in Amer. Chem. Soc. Monograph on New Uses of Sulfur, 1975).

Solid electrolytes are also useful in other electrochemical cells. In the electrowinning of sodium metal from a dissociable molten salt such as NaCl--AlCl₃ or Na_(x) S_(y), Na⁺ ions are electrically transported from the molten salt through the solid electrolyte and converted to elemental sodium at the cathode. In purifying sodium contained in a molten alloy, such as sodium amalgam or other source of impure sodium, sodium ions are produced at the anode and are electrically attracted through a solid electrolyte. The ions are then deposited at the cathode as pure elemental sodium.

Brown et al. U.S. Pat. No. 3,476,602 have disclosed a rechargeable Na-S battery in which the solid electrolyte consists of fine hollow fibers made of glass, glassceramics, etc. Silicate glasses of the formula Na₂ O.3SiO₂, borate glasses of the type Na₂ O.2B₂ O₃ containing up to 0.5 NaF or 0.15 NaCl or NaBr and borosilicate glasses of the type 2Na₂ O.4SiO₂.B₂ O₃ have been proposed for such use. These glasses however are subject to cracking and pitting which greatly reduce the operating life of the battery.

Levine et al., U.S. Pat. No. 3,663,294 propose to increase the operating life of such a battery by using a molten polysulfide catholyte which has a low hydroxide content and thus reduce attack on the glass electrolyte.

The present invention increases the operating life of such a battery by utilizing a special kind of glass which is highly resistant to corrosion under the operating conditions of the battery.

DISCLOSURE OF INVENTION

It has now been discovered that sodium aluminum phosphate glasses with compositions corresponding to points that lie within the quadrilateral ABCD region of the Na₂ O, Al₂ O₃ and P₂ O₅ ternary phase diagram of FIG. 1 where the mole percent of the compositions corresponding to the corners of the quadrilateral are

    ______________________________________                                                Na.sub.2 O                                                                               Al.sub.2 O.sub.3                                                                           P.sub.2 O.sub.5                                   ______________________________________                                         A        53          13          34                                            B        53           5          42                                            C        36          13          51                                            D        36          21          43                                            ______________________________________                                    

as well as such glasses containing up to 11 mole percent of one or more ZrO₂, MgO, and SiO₂, are good ionic conductors and are resistant to corrosion by molten sodium and sulfur at about 300° C. This invention provides a method for conducting sodium ions utilizing a glass of this composition.

This invention also provides an electrochemical device having (a) two electrodes, one of which is a sodium-containing electrode capable of supplying sodium ions, (b) a solid electrolyte separating the two electrodes and (c) an inert connecting electrical connector to complete an electrical circuit between the two electrodes wherein the solid electrolyte consists essentially of the aforementioned sodium ion conducting glasses.

As used in the battery cell, the glass electrolyte can be in the form of thin membranes fabricated in various orientations. These membranes can be in the form of flat plates, corrugated sheets, spirals or other designs which during operating will provide for anode metal ion transfer but will keep separate the liquid anode and cathode materials.

A preferred form for the electrolyte is fine, hollow glass fibers wherein the individual fibers have an outside diameter wall thickness ratio of at least 3, ordinarily from about 3 to about 20 and preferably from about 4 to about 10. Usually within these ratios, fibers having an outside diameter from about 20 to about 1000 microns and a wall thickness of from about 5 to about 100 microns are used. Such hollow fibers provide a high strength, thin walled membrane and give a high ion conductivity. They also provide a very large surface area to volume ratio. Although less advantageous in the latter respect, fibers as large as 5000 microns outside diameter and having walls as thick as 1000 microns can be employed.

For use in a battery cell, the hollow fibers can be fabricated into bundles of circular, rectangular prismatic or other geometric cross-sectional shapes which provide for a controlled orientation and substantially uniform spacing between fibers. The actual fabrication of the electrolyte fibers into a predetermined configuration readily can be carried out by one skilled in the art using known handling, packing and fabricating techniques. To illustrate, bundles of the fibers each having one end closed can be prepared wherein the open ends of the fibers are passed through and sealed into a common header which in turn either serves as or communicates with a reservoir for anode metal.

These hollow fibers can be sealed in place as a bundle in a header, for example, by adhesives such as glazing or potting compounds, solder glass, high temperature thermosetting resins and the like materials.

Preferred are Na-S batteries in which the solid electrolyte consists of sealed hollow fibers which contain sodium.

BRIEF DESCRIPTION OF FIGURE

FIG. 1 shows the ternary phase diagram Na₂ O--Al₂ O₃ --P₂ O₅ and the region ABCD of the diagram which corresponds to the Na₂ O/Al₂ O₃ /P₂ O₅ compositions of the glasses of this invention.

The sodium aluminum phosphate glasses of this invention have substantially Na⁺ conductivity. For example, at 300° C. they have conductivities in excess of 5×10⁻⁵ ohm⁻¹ cm⁻¹ and some have conductivities of the order of 5×10⁻⁴ ohm⁻¹ cm⁻¹. These glasses are also resistant to corrosion by molten sodium at about 300° C. These properties allow these glasses to be used as solid electrolytes in batteries, coulometers, timers, display devices, and in sodium-winning and sodium-purifying cells. (See M. Voinov, "Various Utilization of Solid Electrolytes" in Electron Processes in Solid State Ionics, Proc. Nato. Adv. Study Institute held in Ajaccio, Corsica, Aug. 23-Sept. 9, 1975, edited by M. Kleitz and J. DuPuy, D. Reidel Publ. Co., Boston (1976)).

The mole percent of the three major components Na₂ O, Al₂ O₃ and P₂ O₅ are chosen so as to always total 100%. Therefore when the additives ZrO₂, MgO and SiO₂ are present, the total mole percent exceeds 100%. Preferred are the compositions with approximately 50% Na₂ O, 12% Al₂ O₃, 38% P₂ O₅ and 0-7% of at least one of MgO and SiO₂.

The constituent oxides or their precursors can be used as starting materials to make the glasses. In the examples NaHCO₃, (NH₄)₂ HPO₄ and Al(OH)₃ were used along with MgO and SiO₂. However other starting materials include nitrates, chlorides, sulfates or carbonates. The starting materials are weighed and then thoroughly mixed using, for example, an agate mortar and pestle, a Spex® oscillating-type shaker, or an orbital agate mill. The mixture is then placed in a crucible made from an inert material such as platinum. When smaller size crucibles are used the material sometimes bubbles out when heated to melting temperatures. This can be avoided by first calcining the material at a lower temperature of 500 to 1000° C. for about 2 hours to remove volatiles. The crucible is then placed in a furnace and held at temperatures of the order of 1200-1300° C. until the materials have completely melted. In all compositions of this invention this occurred in less than 16 hours.

After melting, the glass melt is quenched by pouring it onto a steel plate to form a solid glass. In some cases the thermal shock is sufficient to crack the glass into pieces that are too small to test. These samples were repeated and the glass melt slow cooled from an appropriate temperature to remove stresses. The solid glass was then cut or ground to a size convenient for testing.

No water was used on the samples at any time. Any water in or on the glass is likely to react with the molten sodium in any electrochemical device and decrease the corrosion resistance and conductivity of the samples. Lapping oil was used to cut and polish samples and the residual oil was removed with a methanol rinse. The samples were all vacuum baked at or above 150° C. for at least 16 hours prior to any testing.

X-ray diffraction patterns were taken to verify that the product was indeed a glass. A sample of glass was powdered and passed through a 325 mesh screen and submitted for Norelco x-ray diffraction using copper Kα radiation. The detector was run at a sensitivity of 500 counts full scale with a time constant of 3. Chart speed or diffractometer speed was 1° of 2θ/min. Lack of crystallinity was indicated by no recognizable or identifiable peaks in the pattern. To determine conductivity, samples were prepared in the form of a rectangular prism or cylinder, with typical dimensions of about 0.5 cm for easy handling. The sample was mounted in a stainless steel holder between two disks of sodium foil 0.015" thick supported on nickel screens and the sample temperature was raised above the melting point of sodium metal to assure good contact. The a-c conductance was measured using a Wayne-Kerr Universal bridge at a frequency of 10⁴ radians/ second. The dc conductivity was checked by applying slowly varying voltages (triangular wave forms with a frequency less than 0.01 hz) and recording the current. The dc conductivity was calculated from the current-voltage plot. Excellent agreement with the ac results were obtained. Upon removal of the sample, the sodium was cleaned from the glass and no apparent attack by the molten sodium was evident. Final traces of sodium were removed by using a methanol rinse.

In order to further verify resistance to sodium corrosion, glass samples were weighed to four significant figures and then placed in quartz tubes. The tubes were loaded with molten sodium at 280° C. and the glass samples allowed to soak in the molten sodium for 48 hours. The samples were cooled and rinsed with methanol to remove excess sodium. The samples were then weighed and were judged to have sufficient sodium corrosion resistance if the weight loss was less than 0.3 wt. %. In some cases sodium was not removed with the standard methanol rinse. In those cases an ultrasonic rinse was used for 10 minutes to remove the excess sodium. A sodium build-up less than 0.5 wt. % was judged not detrimental. The glasses of this invention fall within these limits of weight loss.

The glasses of this invention are useful as the solid electrolytes in electrochemical devices having two electrodes one of which is a sodium-containing electrode capable of supplying sodium ions. Since glasses are readily cast in a variety of forms, the glasses of this invention are especially useful in devices in which the solid electrolytes must be in a form which is difficult to make with non-glasses, e.g., the fine hollow tubes of the Brown U.S. Pat. No. 3,476,602 batteries.

BEST MODE

The best mode of practicing the invention is exemplified by Example 2.

EXAMPLES 1-3

The amounts of starting materials NaHCO₃, (NH₄)₂ HPO₄, Al(OH)₃ and MgO and SiO₂ used in preparation of each of these Examples are shown in Table I. The starting materials were thoroughly mixed and then placed in a platinum crucible. If the mixture was calcined before melting, the calcining temperature is also indicated as in Table II. Calcining time was about 2 hours. The crucible and contents were heated to the melting temperature given in Table II and held at this temperature until the materials had melted completely. In all examples the time required was less than 16 hours. The glass melt was poured onto a steel plate to quench it into a solid piece.

X-ray diffraction measurements were done on the product to verify that they were indeed glasses, i.e., no identifiable peaks were found. The mole percent of each of the constituent oxides is given in Table III and the results of Na⁺ conductivity and corrosion resistance measurements described above are given in Table IV.

                  TABLE I                                                          ______________________________________                                         Starting Materials - grams                                                     Example                                                                               NaHCO.sub.3                                                                             (NH.sub.4).sub.2 HPO.sub.4                                                                 Al(OH).sub.3                                                                          MgO   SiO.sub.2                             ______________________________________                                         1      16.69    20.08       3.6259 0.1703                                                                               0.5078                                2       18.228   21.793     4.057  --    --                                    3      13.32    21.43       4.4167 0.156 0.5239                                ______________________________________                                    

                  TABLE II                                                         ______________________________________                                         Example  Calcine Temp. ° C.                                                                      Melting Temp ° C.                              ______________________________________                                         1        500             1200                                                  2        900             1300                                                  3        --              1200                                                  ______________________________________                                    

                  TABLE III                                                        ______________________________________                                         Constituent Oxide - Mole Percent                                               Example Na.sub.2 O                                                                               P.sub.2 O.sub.5                                                                        Al.sub.2 O.sub.3                                                                       MgO   SiO.sub.2                              ______________________________________                                         1       50        38      12      2     4                                      2       50        38      12      --    --                                     3       42        43      15      2.1   4.6                                    ______________________________________                                    

                  TABLE IV                                                         ______________________________________                                                 % Weight         .sup.σ Na.sup.+                                 Example Change - Corrosion Test                                                                         300° C. ohm.sup.-1 cm.sup.-1                   ______________________________________                                         1       -.01             480 × 10.sup.-6                                 2       +0.28            430 × 10.sup.-6                                 3       0                130 × 10.sup.-6                                 ______________________________________                                     

I claim:
 1. Method comprising electrically conducting sodium ions through a glass whose components lie within the area of the quadrilateral ABCD of the Na₂ O, Al₂ O₃ and P₂ O₅ ternary phase diagram of FIG. 1 where the points A, B, C and D are defined by the following mole percentages:

    ______________________________________                                                Na.sub.2 O                                                                               Al.sub.2 O.sub.3                                                                           P.sub.2 O.sub.5                                   ______________________________________                                         A        53          13          34                                            B        53           5          42                                            C        36          13          51                                            D        36          21          43                                            ______________________________________                                    

the mole percent of the components within the quadrilateral totaling 100%, said glass containing 0 to 11 mole percent of an oxide component selected from the group consisting of ZrO₂, MgO and SiO₂.
 2. The method of claim 1 wherein the glass has the mole percentages of Na₂ O, Al₂ O₃ and P₂ O₅ of 53, 13 and 34 respectively.
 3. The method of claim 1 wherein the glass has the mole percentages of Na₂ O, Al₂ O₃ and P₂ O₅ of 53, 5 and 42 respectively.
 4. The method of claim 1 wherein the glass has the mole percentages of Na₂ O, Al₂ O₃ and P₂ O₅ of 36, 13 and 51 respectively.
 5. The method of claim 1 wherein the glass has the mole percentages of Na₂ O, Al₂ O₃ and P₂ O₅ of 36, 21 and 43 respectively.
 6. The method of claim 1 wherein the glass contains ZrO₂.
 7. The method of claim 1 wherein the glass contains MgO.
 8. The method of claim 1 wherein the glass contains SiO₂.
 9. The method of claim 1 wherein the glass has the following mole percentages: Na₂ O about 50; Al₂ O₃ about 12; P₂ O₅ about 38; and 0 to 7 mole percent of one or more of MgO and SiO₂.
 10. The method of claim 9 wherein the glass contains no MgO or SiO₂.
 11. The method of claim 9 wherein the glass contains 2 mole percent of MgO and 4 mole percent of SiO₂.
 12. The method of claim 1 wherein the glass has 42 mole percent of Na₂ O, 43 mole percent of P₂ O₅, 15 mole percent of Al₂ O₃, 2.1 mole percent of MgO and 4.6 mole percent of SiO₂.
 13. In an electrochemical device having (a) two electrodes, one of which is a sodiumcontaining electrode capable of supplying sodium ions, (b) a solid electrolyte separating the two electrodes, and (c) an inert connecting electrical connector to complete an electrical circuit between the two electrodes, the improvement wherein the solid electrolyte consists essentially of a sodium ion conducting glass whose components lie within the area of the quadrilateral ABCD of the Na₂ O, Al₂ O₃ and P₂ O₅ ternary phase diagram of FIG. 1 where the points A, B, C and D are defined by the following mole percentages:

    ______________________________________                                                Na.sub.2 O                                                                               Al.sub.2 O.sub.3                                                                           P.sub.2 O.sub.5                                   ______________________________________                                         A        53          13          34                                            B        53           5          42                                            C        36          13          51                                            D        36          21          43                                            ______________________________________                                    

the mole percent of the components within the quadrilateral total 100%, said glass containing 0 to 11 mole percent of an oxide component selected from the group consisting of ZrO₂, MgO and SiO₂.
 14. The device of claim 13 wherein the conducting glass has mole percentages of Na₂ O, Al₂ O₃ and P₂ O₅ of 53, 13 and 34 respectively.
 15. The device of claim 13 wherein the conducting glass has mole percentages of Na₂ O, Al₂ O₃ and P₂ O₅ of 53, 5 and 42 respectively.
 16. The device of claim 13 wherein the conducting glass has mole percentages of Na₂ O, Al₂ O₃ and P₂ O₅ of 36, 13 and 51 respectively.
 17. The device of claim 13 wherein the conducting glass has mole percentages of Na₂ O, Al₂ O₃ and P₂ O₅ of 36, 21 and 43 respectively.
 18. The device of claim 13 wherein the conducting glass contains ZrO₂.
 19. The device of claim 13 wherein the conducting glass contains MgO.
 20. The device of claim 13 wherein the conducting glass contains SiO₂.
 21. The device of claim 13 wherein the conducting glass has the following mole percentages: Na₂ O about 50; Al₂ O₃ about 12; P₂ O₅ about 38; and 0 to 7 mole percent of one or more of MgO and SiO₂.
 22. The device of claim 21 wherein the glass contains no MgO or SiO₂.
 23. The device of claim 21 wherein the glass contains 2 mole percent of MgO and 4 mole percent of SiO₂.
 24. The device of claim 13 wherein the glass has 42 mole percent of Na₂ O, 43 mole percent of P₂ O₅, 15mole percent of Al₂ O₃, 2.1 mole percent of MgO and 4.6 mole percent of SiO₂.
 25. The device of claim 13 which is an electrolytic cell.
 26. The device of claim 13 which is a galvanic cell.
 27. The device of claim 13 wherein the anode is sodium and the cathode is sulfur.
 28. The device of claim 13 wherein the solid electrolyte is in the form of sealed hollow fibers containing sodium as anode and the cathode is sulfur.
 29. The galvanic cell of claim 26 having the solid electrolyte in the form of hollow fibers containing molten sodium as anode, and molten sulfur is the cathode. 